This invention relates to an ultrasonic detection system having an array of ultrasonic transducers for forming transmission and/or a reception beams by properly choosing a group of transducers and/or respectively applying appropriate phase-shifts to the signals to be fed to each of the transducers or to the signals produced from each of the transducers. More particularly, the invention relates to an ultrasonic detection system which suppresses and maintains sub-beams or sidelobes much smaller with respect to a main transmission or a reception beam compared to prior art systems.
This invention has a particular application in a so-called electronically scanning sonar system which is installed on a ship and used for observing environmental underwater conditions by detecting targets of interest such as schools of fish and the seabed and by displaying the returned signals in the indicator of the system.
As well known, there are various kinds of arrays of ultrasonic transducers available for ultrasonic detection systems. These include, for example: (1) a linear array wherein a plurality of transducers are closely and uniformly disposed at the same spacing intervals in such a manner that the radiation or reception surface of each of the transducers is maintained on a straight line; (2) a non-linear array including a circular one wherein a plurality of transducers are closely and uniformly disposed on a curved line; (3) a planar array wherein a plurality of transducers are closely and uniformly disposed in a plane; (4) a non-planar array wherein a plurality of transducers are closely and uniformly disposed in a curved plane; and (5) a cylindrical array wherein a plurality of transducers are closely and uniformly spaced and arranged in such a way that their radiation or reception surfaces are maintained touching the inside of the periphery of a cylinder.
Hereinafter, this invention will be explained as embodied in an electronically scanning sonar system. The technique for forming a reception beam with a linear array of ultrasonic transducers is, for example, well known. Assuming that all the receiving transducers of an array are simultaneously energized, echo signals are sensed by the transducers coming back from a direction perpendicular to the line of transducers, thus, forming a reception beam in that direction. The reason that a directional reception beam is formed in the perpendicular direction is that the echo signals returned from the direction algebraically add up to re-enforce each other. However, echo signals from other directions are algebraically added to cancel each other.
Also well known is that when, for example, a pluraltiy of receiving transducers of a linear array are simultaneously energized, sidelobes pointing in undesirable directions are inevitably produced on both sides of a main reception beam pointing in a specific searching direction. If these sidelobes are relatively large with respect to a main beam, the echo signals from other undesirable directions as well as from a specific searching direction are received and indicated over the PPI display. Alternatively, false signals are indicated thereover in directions where no targets actually exist. Thus, large sidelobes make it extremely difficult to obtain the signals accurately representing environmental underwater conditions, and, hence, are a significant cause of operator error in distinguishing the signals from desired targets in an indicator.
Accordingly, it is highly desirable that sidelobes should be suppressed to the maximum extent possible and maintained as small as possible with respect to a main beam.
The same kind of problems also arise in transmitting ultrasonic wave energy in a beam form by simultaneously exciting a plurality of transducers of a linear array. If large sidelobes are formed on both sides of a main transmission beam pointing in undesirable directions, the echo signals resulting from transmitted energy come back from undesirable directions as well as a searching direction. This makes it difficult to obtain the signals accurately representing environmental underwater conditions in the reception mode. In addition, large sidelobes cause much radiation energy to be wasted, and, hence, energy management of the overall system is not efficiently maintained.
The same technical problems also arise when any one of the non-linear, circular, planar, non-planar and cylindrical array of ultrasonic transducers is used to form reception or transmission beams.
Magnetostrictive as well as electrostrictive ultrasonic transducers are used to form any kind of array of ultrasonic transducers.